Relating to the testing of insulation.



S. EVERSHED. RELATING TO THE TESTING OF INSULATION. APPLICATION FILED use. a0. 1915.

1,296, 1 75,. Patented Mar. 4, 1919.

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Wnesses: v fli zl ww S. EVERSHED. RELATING TO THE TESTING OF INSULATION:

I APPLICATION FILED-DEC. 30, I915. 1,296,175. I Patented Mar. 4,1919.

SHEETS-SHEET 2- 5 ea I I I I I I I I I III IIIIIII I I IIIIIIW II/lfiwesses: 1 E I Invanfior;

J Au-Z; @dae y-@enskeah UNITED STATES PATIENT OFFICE.-

SYDNEY em wlel, Lenee raeueemm nELArme' :po zrnsrme or I SULAT ON.

specificatiomof Letters'I'Eatent.

Patented Mar. 4,. 1919.

Original application fiIed March- QI, 1914, Serial mTo. 826, 372r Divided and this application fi1 ed December =30, 1915. Seria1No.-69,466.

explained it has beenlcno-Wn tornmany. years that the resistance of insulation .undeitordig nary Working conditions is usually good deal less. at a high voltage than it isat. a low voltage, and that I have discoveredsby ;experimental researchtone'ofthecauses: ofthis. I have-then explained in considerable .detail how the invention in thatapplication consists in a method-of diagnosis for ascertaining the condition .of insulationby ,actual or implied reference to ,the avhole orany significant partaofthe characteristic curve deduced from tests of the; resistance or conductance 0f the insulation at two or-mQre suitably chosen and different voltages.

As the principle, underlying that inven! tion also underlies the present invention reference should :be. made. to the abovel numb d app ca n for detai s-the eei- In anothe application filed-.ojievendate i th presen applieaten, ,I. :hev -d Y scribed appa a u l"m Z Il 1 g-?J. fil te tdifferences, ,as-for example, Whenit is in.-

tended to ascertain theposition of the point or region of i-nflexion, .then n carry ng out the invention the ,testlng apparatus is arranged and .adapted.-.to facilitate the .compa-rison of .one Jdifierece with another, :preferably :by. means :.0fmadjustableurheostats (herein referred to as fdecrement :rheostats). adapted to. rivedirect indications-of the ,diflerencesin the resistance :(or conductance) of the insulation, wcorresponding:to successive increments 20f testing pressure.

Such apparatus ;has also been .dealtwith generallyhr the application filed herewith, but the-present caseldeals with it specifically. The: present invention, therefore in brief consists in apparatus adapted/for the investigation ofany partaof the characteristi curve of insulation, momprising in .cqmbina tion an instrument for measuring-resistance e d etanee.) adaptedabyameans :OfuJtW or more decrement rheostats, or equivalent means; to indicate resistanee.: fi ren ee .(jor con u an differences), a d a sour e eleetro-motive force suited to give the re.- qulredseries of testing- ;pressulfes.

As has beenexp a nei avhe theobjeet ef the tests is to investigate some assigned p011- t1on of the characteristic curvea .oomp arisen of tdifierences maybe made. "For example, =to a c t in wh a a y. ssig ed voltag E, he arac ri ti cu h r a hed o pass d beyond the point or region of inflexion, it is uff cient to.t tt11e..insu1a ien. at Pressures namely Ee, E, and ;E-{.,e y ol ts and to a c rta t c r espond ng re is valuess ya, Z2, d .6, r spec ve y: i it is 2 .01 .11 tha ees ne -erased a1 he curv h not been ar ie -b ams he po of iefi x e zby'subiee n-g: the n ul en t th Pre s re-E ThQs wh r-e a qua nt l W thath .mae eme eezl {theor of curves will understand that this process of ee pa -is b s upon he i ettha a a Po f nfleze en e f/ 3 he i smin inum alue and d ft/ .1 e ang et s gn enm g th ough m et cha a ter st curves of insulation thepoint .of inflexion is somewhat ill defined, being a. reg ion .of infiexion rather than a point,and it is therefore generally impracticable to I make the necessary test with small increments of pres.-

u e. I pra t it Wi l o t be n ce sary to make the difference evolts as much-as one fifth or even one fourththe value oftheassigned pressure E, in order to 'insure a -measunable difference in the corresponding resistance values. Hence the difi'erentialsensibility of the measuring instrument should be as great as possible, and for carrying out the process of comparison of differences a VVheatstone bridge or a difi'erential galvanometer is more suitable than a galvanometer or an ohmmeter of ordinary type.

In carrying out this part of the invention the preferable type of measuring instrument is a VVheatstone bridge. Inone form the bridge has ratio arms capableof adjustment to any value over a wide range in order that insulation resistances of widely different magnitudes may be balanced. The resist ance in the third arm, corresponding with, the adjustable arm of an ordinary bridge, is dividedinto two sections, each of which is separately adjustable and so arranged as regards the marking and figuring of the resistance values that the scale reading or figures indicate the decrement or amount of resist ance withdrawn or'subtratced from the total amount initially contained in the third arm the inverse of the ordinary arrangement of a rheostat, which would be figured to indicate the amount of resistance added.

In carrying out the invention subject to the remarks made above, the measuring instrument or appliance may be of any known type;-'

In carrying out the invention the measuring instrument or appliance may be of any known type uitable for the measurement of insulation resistance or insulation conductance, but although the properties of insulators may be expressed in terms of either resistance or conductance, it has so longbeen the'custom to regard resistance as the criterion of insulation that in describing the invention in detail it will be assumed that the testing apparatus is required to determine characteristic curves in terms of voltage and resistance. As a matter of convenience in testing the indicating instrument may be so arranged that insulation resistance or conductance may be measured in the customary units whenever so desired without in any waylimiting the power to indicate resistance difl'erences at will for the purpose of obtaining results which are readily comparable. Y

Y The necessary testing pressures may be provided by a'battery or alternatively by a dynamo. or a magneto generator of direct current type, either hand driven or power driven as may be convenient, and adapted by means of multiple voltage devices to give the required series of testing pressures. A series of from ten to twelve premures, suitably graded, will generally enable the complete characteristic curve to be investigated or drawn up to the breakdown point if the maximum pressure of the series is sufficient for the purpose. A series of six to eight pressures w1ll generally-sufiice to draw the first part of the characeristic curve from about 50 volts up to the point of infiexion age of the system for examplethen a series of three pressures will be enough; Any

known principle either alone or in combination may be uesd for obtaining the multiple voltages required by my method of testing, as for example by a sub-divided battery, or by varying the speed of a dynamo, or by varying the induction or the number of effective turns of. wire in'the armature of a dynamo, or by a number of windings on a single armature each connected to one of a corresponding number-10f commutators, or by a multiple. armature each element having its appropriate winding. 1

In orderithat my invention may be better understood it willnow be described with reference to the accompanying drawings in which:

Figure 1 shows an arrangement according to one form of the present invention employing a Wheatstone bridge, and r Fig. 2 a modification utilizing a differential galvanometer. a In Fig. 1,0 andb are the ratio Wheatstone bridge of which the rheostat V and decrement rheostats D and D form the adjustable arm. A decrement rheostat is a rheostat in which the scale is figured so that the zero corresponds to the maximum resistance when the conductor circuit is closed, and lesser values of the resistance are indicated by figures which respectively give the difference or decrement of the corre' sponding resistance from the said maximum reslstance. The indrcatlng instrument G arms of a may be a sensitive galvanometer of any known type which is suitable for use with a bridge. The testing pressures may be supplied from any convenient ource; by way of example a two-component generator having two armatures X and Y is shown, and their component pressures are algebraically added by a permutation switchhaving two mem bers P and P In the present invention the armature Y may be wound to genera-tea pressure equal to'the working voltage,-say E volts, and the armature X may be wound to generate a fractionof that pressure, say 6. volts. For example if E, the working voltage, be 500 volts then e might very conveniently be made equal to 100 volts.

'In Fig. l the permutation switch is shown setto the second voltage point whichgives Ee volts as the testing pressure. At this pressure the bridge has been balanced by first adjusting the ratio arm b so that it bears a sufiiciently small ratio to a, and then obtaining an accurate balance by adjusting the rheostat, Vthe decrement switches having been initially set so that eachfstandsat zero scale reading, corresponding'with maximum resistance. The permutation switch would out my invention by means of voltage-conductancecurvesinstead of voltage-resistance 7 curves the appropriate instruments would be modified, where necessary, to indicate con-I ,ductance units. I A Wl1eatstone bridge or a differential galvanometer, would need no modifications except in the rheostats in the adjustable third arm, Which would be com-v posed of a number of unit'resistances adapted so that any desired number could be con-v nected up in parallel to give any total value of conductance. These and other similar inversions either necessary or convenient for the purpose of eonvertln -res1stance measur- 1 ing apparatus into con uctance measuring apparatus will be readily understood 'by those who are conversant with the art of electrical measurement. r

It is to be understood that? in making measurements to determine two or more points on a characteristic curve the tests must proceedby ascending voltages from the lowest required testing pressure upward step avoided.

by step to the highest pressureof the required series, in order that hysteresis effects, which are a marked feature of conduction through absorbent insulators, may be .Havingnow described my invention, what I claim as new and desire to secure by Letters :Patent is 'ing pressures.

1... Apparatus for the investigation of any,

part of the characteristic curve of insulation, comprising in combination a VWheatstone bridge with variable ratio arms and having its fourth arm adapted to receive the test element, a rheostat inthe third adjustable arm of the bridge adjustable to bal-,

ance the resistance of the test element under the lowest initial value of the series of test-. ing pressures, a plurality of decrement rheostats forming part of the circuit of the adjustable arm .of the bridge all initially set at the Zero of their respective; scales and each in turn being adjusted to balance in combination with the previously adjusted resistances the resistance of the test element at one value of the; series of testing pressures, and a source of eleotromotive force adapted to give the required series of test-:

1 2. Apparatus for theinvestigation of any I part'of theoharaoteristic curve of insulation,

comprising in combination a Wheatstone bridge with varlableratlo arms and havlng its fourth arm adapted to receive the test element, a rheostat in the third adjustable arm of the bridge adjustable to balance the tion with the previously adjusted resistances the resistance-of the test; element at one value of the series of testingpressures,

a scale co-mmon to the said decrementrheostats, and a source 0t electromotive force adapted togive the required series of test ing pressures; 7 1 wv 7 o 3. Apparatus for the investigation of any part of thecharacteristic curve of insulation, comprising in combination azdifierential galvanometer having twodifierentially wound coils which carry the leakage cur rent and the balancing current respectively,

a rheostat in the balancing circuit adjust able to balance the resistance of the test ele ment under the lowest initial value of'the,

series of testing pressures, a' pluralityiof decrement rheostats in the balancing circuit all. initially set at thezeros of, their respective scales and ,each in turn being adjustedv to balance in combination with the previously adjustedresistances the resist,,

.ance of the test element at one value of the series of testing pressures, and a source of electromotive force adaptedto give'the re. quired series of testing; pressures. i 4c, Apparatus-for the investigation of an part of the characteristic curve of insu1a tion .com r1s1n 1n combinatlon a differential galvanometer having two difierentially wound coils which carry'the leakage ,currentfrom the test element and the balancing current respectively, an. adjustable shunt in parallel with the balancing current coil of the galvanometer, arheostat in the balancing circuit adjustable to balance the resistance of the test element under the lowest initial value of the series of testing pressures, a pluralityiof decrement rheostats in the balancing circuit all initially set at the zeros oftheir respective scales and each in turn being adjusted to balance in combination with the previously adj usted resistances the resistance {of the test element at one 7 value of the series of testing pressures, and

- a source of electromotive force adapted to give the required series of-testing pressures. 7

,5. Apparatus for the investigation of any part of the characteristic curve of insulation, comprisingin combination a differential galvan-ometer having two .diflerentially wound coils which carry the leakage current from the test element and thebalancing current respectively, a rheostat in the balancing circuitadjustaible to balance thev resistance of thetest element under the lowest initial value of the series of testing pres sures, a plurality of decrement rheostats in the balancing circuit all initi'allyset at the Zeros of their respective scales and each in turn being adjusted tobalanoe' in combinae tion with the previously adjusted resistances the resistance of the test element at one value of the series of testing pressures, a

scale common tothesaiddecrement rheo stats, and, a sourceof electromotive force adapted to give the required series of testing pressures.

6. Apparatus for the investigation of any part of the characteristic curve of insulation, comprising in combination a difi'erential galvanometer having two differentially Wound coils which carry the leakage current from the test element and the balancing current respectively, an adjustable shunt in parallel With the balancing current coil of the galvanometer, a rheostat in the balancing circuit adjustable to balance the resistance of the test element under the lowest initial value of the series of testing pressiu'es, a plurality of decrement rheostats in the balancing circuit allinitially set at the zeros of their respective scales and each in turn being adjusted to balance in combination with the previously adjusted resistances the resistance of the test element at one value of the series of testing pressures, a scale common to the said decrement rheostats, and a source of electromotive force adapted to give the required series of testing pressures. v

In testimony whereof, I have afiixed my signature.

SYDNEY EVERSHED,

Copies of this patent may be obtained for five cents each, by addressing the "Commissioner of Iatents,

Washington, D. 0. 

